:: The second century of the metric system ::

The years 1860-1870 announced and inaugurated a period of renewal for the metric system. All scientists recommended its use, like the Englishman Maxwell or the German Grauss. After the 1867 International Exhibition, a Committe stated that "the metric system can totally be adopted…in science, art, industry and trade". In 1870, it was the only legal system in thirteen countries, it was the basis to standards of eight other countries, and its use was legally allowed in the majority of the others, from which the United States.

At a metrological level, a decimal system based on the Archives standards was recommended. In August 1870, at the French government's request, a "Metre International Commission" met in Paris. Because of the war, it only appointed a preparatory committee. Works were resumed in 1872 with the participation of twenty-eight countries. On 20 May 1875, the representatives of seventeen countries, out of the twenty countries that were present, signed the "Metre convention".

This Convention created a set of organisms entrusted with ensuring the uniformity of physical measures worldwide. These organisms are the "International Bureau of Weights and Measures" (IBWM), a permanent metrology laboratory situated in Sevres, near Paris; the "International Committee for Weights and Measures" (ICWM) which controls directly the IBWM and prepares decisions and recommendations; lastly, the "General Conference of Weights and Measures" (GCWM), a higher jurisdiction that calls together the representatives of the Convention signatory states every fourth year.

The first General Conference held in 1889 sanctionned the realization of international prototypes that were "deduced from the values of the Legal Metre and Kilogramme, as they stand at present". These prototypes are made of an alloy of platinum and ten per cent of iridium.

They were made with drastic precautions which would have been unthinkable a century earlier. The Legal standards platinum had many impurities. It was made up of platinum powder that was agglomerated by forging. The new alloy was melted; it was analyzed carefully to check that there were no oxidizable or magnetic impurities like iron or ruthenium. The new standards were produced between 1871 and 1888, firstly under the responsibility of the French Section of the Metre International Commission, and especially of the chemist H. Sainte-Claire, of Deville and of the physicist H.Fizeau who themselves carried out many checks, then under the responsibility of the ICWM and the IBWM.

The 1889 international Kilogramme is a cylindre. It is still nowadays the mass prototype. The 1889 international Metre is a x-section ruler. It is 102 cm long, and has a line at one centimetre from each end. The distance between these two lines define the metre.

The practical metre length is still the one given by this prototype, even for measures that demand a high degree of accuracy.

Mètre et kilogramme étalons of 1889 (IBWM)

However, the accuracy allowed by this material standard has became inadequate for some present needs, and it was thought necessary to modify the metre's definition, by adopting an "optical standard". A wavelength was chosen as a basis in 1960. The definition changed again in 1983 when it was linked to the speed of light.

Nowadays, another two international organisations work towards the implementation - at pratical and legal levels - of the GCWM. Theses organisations are the "International Organisation of Legal Metrology" (IOLM), in charge of the international harmonization of legislation relating to units of measure and the "International Standards Organization" (ISO) in charge of standardizing the use and the writing of the SI units symbols - SI standing for International System of Units.

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